1. Field of the Invention
The present invention relates to an object driving apparatus having a spring and an ultrasonic motor and, more particularly, to an object driving arrangement suitable for use as a shutter system for an optical apparatus such as a camera.
2. Description of the Related Art
Various types of shutter systems for optical apparatuses such as cameras have heretofore been known. One conventional example of the shutter systems is arranged to charge a shutter driving spring via a gear, a cam or a lever by means of a known electromagnetic motor before the start of a running of a shutter curtain and then holds the spring in its charged state by means of a retaining mechanism for suppressing the action of the driving spring. In this example, if the running of the shutter curtain is to be started, the retention of the spring by the retaining mechanism is cancelled by a electromagnet, a cam or the like, so that the shutter curtain is driven by the driving force generated by the spring.
The aforesaid shutter system has an arrangement such as that shown in FIG. 16. The output speed of a single charging driving source which is not shown (for example, an electromagnetic motor) is reduced to drive a first charging lever 70, thereby approximately simultaneously charging leading and trailing curtains via a second charging lever 71.
However, the above-described shutter system has the following problems.
(i) Since a known electromagnetic motor is employed as a driving source for charging a shutter curtain driving spring, a gear mechanism of large speed reduction ratio and a power transmission mechanism, such as a cam or a lever, must be disposed between the motor and the spring in order to obtain a sufficient torque to elastically deform the spring. Of necessity, the complexity and size of the mechanical structure of the shutter system increase and the manufacturing cost thereof increases.
(ii) Running members integral with the respective shutter curtains (shutter-curtain holding frames) strike against stopper members at high speeds at their shutter curtain running completion positions, with the result that a shock noise or a disagreeable vibration occurs.
(iii) It is impossible to stop the shutter curtains halfway in their running paths while they are running toward the respective stopper members, to cause each of the shutter curtains to run at various speeds, or to reduce the speed of each of the shutter curtains during running.
(iv) As the shutter speed of the shutter system is made higher, the running shock of the shutter curtains becomes larger. This leads to the problem that not only is a disagreeable vibration or noise caused but also a camera shake occurs. In general, since the curtain speed of the shutter system is adjusted so that its maximum shutter speed can be achieved, the shutter curtains run at the high speed even during a low shutter-speed operation which needs no such high curtain speed, with the result that the running shock becomes far larger. In addition, because of the low shutter-speed operation, a large camera shake occurs compared to a high shutter-speed operation.
(v) In the above-described conventional example, a large speed reduction ratio is adopted so that a shutter charging operation can be securely performed even if the voltage of a power source battery is low. For this reason, even if the battery voltage is high so that it is possible to obtain battery power greater than that required for the shutter charging operation, it is impossible to increase the speed of the shutter charging operation, so that it is impossible to increase the frame speed of the camera.